Typically, printing presses have a modular construction and comprise several stations which perform different operations on the web. For example, the first station creates an inked impression on the web, the second station prints page numbers, the third station perforates the web to create transverse tear lines, etc. For a proper operation of the press, all the modules are synchronised and driven by a single prime mover through a system of gears. By this arrangement, the angular positions of the various cylinders in rolling contact with the web evolve in strict registry one relatively to the other.
During a production run, routine maintenance procedures require to clean one or more of the rotary cylinders of the press. This is achieved by disengaging from the drive the modules that do not need maintenance, while the modules in need of service are operated to rotate the cylinders and wash them with the appropriate cleaning agent. When the cleaning operation is completed, the disengaged modules are reengaged back with the drive and the operation of the press is resumed. It is critical that the re-engagement occurs in such a way as to maintain the pre-established registry between the various press modules. If the registry is lost, the printing operation cannot be carried out.